This should work, but does not:

int main(void)
{
YAML::Node nn;
nn[std::string(2000, 'a')] = 0;
YAML::Load(YAML::Dump(nn));
return 0;
}

--> Terminate called after throwing an instance of 'YAML::ParserException'
what(): yaml-cpp: error at line 1, column 2001: illegal map value

I believe the problem is that the long key should be emitted as such:

? LONGKEY
: value